Eubacteria export numerous proteins across the plasma membrane into either the periplasmic space (Gram-negative species), or the growth medium (Gram-positive species). The Gram-positive eubacterium Bacillus subtilis and, in particular, its close relatives Bacillus amyloliquefaciens and Bacillus licheniformis are well known for their high capacity to secrete proteins (at gram per liter concentrations) into the medium. This property, which allows the efficient separation of (secreted) proteins from the bulk cytoplasmic protein complement, has led to the commercial exploitation of the latter bacilli as important “cell factories.” Despite their high capacity to secrete proteins of Gram-positive origin, the secretion of recombinant proteins from Gram-negative eubacterial or eukaryotic origin by Bacillus species is often inefficient.
General strategies for the secretion of heterologous proteins by bacilli are based on the in-frame fusion of the respective protein with an amino-terminal signal peptide that directs this protein into a secretion pathway, for example the Sec-dependent secretory pathway. Upon translocation across the membrane, the signal peptide is removed by a signal peptidase, which is a prerequisite for the release of the translocated protein from the membrane, and its secretion into the medium.
Proteolysis in bacteria serves to rid the cell of abnormal, and misfolded proteins. A unique mechanism for the destruction of abnormal proteins resulting from abortive termination of translation is provided by the SsrA-mediated tagging and degradation system (for a recent review, see Karzai et al. (2000). The SsrA-SmpB system for protein tagging, directed degradation and ribosome rescue. Nat. Struct. Biol. 7:449–455.). SsrA, also called 10Sa RNA or tmRNA is a highly conserved RNA molecule in eubacteria. It is a unique molecule that can act as both a tRNA and an mRNA in a process referred to as trans-translation (Atkins et al. 1996. A case for trans translation. Nature 379:769–771, Jentsch 1996. When proteins receive deadly messages at birth. Science 271:955–956, Keiler et al. 1996. Role of a peptide tagging system in degradation of proteins synthesized from damaged messenger RNA. Science 271:990–993). This mechanism provides the cell a way to release ribosomes that are stalled on untranslatable mRNAs, e.g. mRNAs lacking in-frame stop codons. In the model for SsrA action, SsrA charged with alanine enters the A site of a stalled ribosome, mimicking a tRNA. The alanine is added to the uncompleted polypeptide chain; and then, serving as an mRNA, SsrA provides a short reading frame followed by a stop codon as a template to add a short peptide to the nascent polypeptide before translation terminates and a tagged protein is released. The peptide tag (encoded by SsrA) functions as a proteolytic degradation signal, and in Escherichia coli four proteases have been identified that degrade proteins tagged by SsrA. ClpXP, ClpAP, and FtsH (HflB) degrade SsrA tagged proteins in the cytoplasm (Gottesman et al. 1998. The ClpXP and ClpAP proteases degrade proteins with carboxy-terminal peptide tails added by the SsrA-tagging system. Genes Dev. 12:1338–1347, Herman et al. 1998. Degradation of carboxy-terminal-tagged cytoplasmic proteins by the Escherichia coli protease HflB (FtsH). Genes Dev. 12:1348–1355), while SsrA tagged proteins with signal peptides that are exported to the periplasm of E. coli are degraded by Tsp (Prc) protease (Keiler et al. 1996).
Protein production and secretion from Bacillus species is a major production tool with a market of over $1 billion per year. However, proteolysis of proteins by endogenous proteases diminishes the production capability of these microorganisms. Thus, it would be beneficial to have an mechanism for the enhanced production and secretion proteins. The present invention provides such an advantage by changing the nonpolar C-terminus of a protein by adding charged, polar residues (or by replacing amino acids), so that the proteins are protected against the bacillus proteases that degrade SsrA-tagged proteins.